PC keeps losing visual

I’m working on my father’s PC as he keeps losing visual. He has a E-machine T6538 desktop. AMD Athlon 64, 512 memeory , 200 gig hard drive, with a built in Nvidia video card at 256mb. I have ran disk cleanup, defrag, Ccleaner, registry mechanic, updated video card driver and direct x, cut down his start up applications in msconfig to the bare min checked for viruses and spyware but still nothing. The pc seems to lose visual often but is still running. Sometimes if I wait it will come back other times it wont and I have to reboot, and that does not always bring it back up. I have replaced the moniter and tried it on mine as well and get the same results. Any help would be great. Thanks in advance.